'Poach' is a much gentler process than 'boil', used for more delicate foodstuffs. You boil eggs in their shells, because the shells are tough and can withstand the violence of fast boiling water. However, if you break the egg first, and cook just the yolk and white, this is known as poaching. You obviously need to treat it much more carefully in this situation. Poaching can sometimes involve a special receptacle inside the water. Similarly, you boil potatoes, but poach fruit such as pears. A peeled pear is quite delicate, so it needs to be cooked in a gentle way.

The difference between poaching and boiling is just a matter of temperature degrees.